The invention relates to a control device and a method for operating an electrical machine driven by an inverter.
For the drive in hybrid or electric vehicles, use is generally made of electrical machines in the form of polyphase machines which are operated in conjunction with inverters. In this case, the electrical machines are optionally operated in the motor or generator mode. In the motor mode, the electrical machine generates a drive torque which, in the case of use in a hybrid vehicle, supports an internal combustion engine, for example in an acceleration phase. In the generator mode, the electrical machine generates electrical energy which is stored in an energy store, such as, for example, a battery or a super-cab. Operating mode and power of the electrical machine are set via the inverter.
Known inverters comprise a series of switching elements used for optionally switching the individual phases of the electrical machine relative to a high supply voltage potential or relative to a low supply voltage potential. Two series-connected switching elements here in each case form a half-bridge branch, wherein a first switching element is connected to the high supply voltage potential and a second switching element is connected to the low supply voltage potential. Each phase of the electrical machine is then connected to a respective half-bridge branch. The switching elements are controlled by an external control unit, which calculates a desired operating point for the electrical machine depending on the driver's desire (acceleration or braking) The inverter is connected to the control unit and receives the corresponding operating data or control commands from it.
In the case of electrical machines which are controlled via an inverter, the semiconductor power components of the inverter—designated hereinafter as power components for short—are subjected to a high thermal loading primarily during operation with high currents at low rotational speeds. In this regard, e.g. electric vehicles or hybrid vehicles in the purely electric driving mode must be able to start exclusively with the aid of the electrical machine. If the electrical machine in this case is connected to the drive and thus the wheels of the vehicle without a rotational-speed-compensating coupling element, such as e.g. a torque converter or a friction clutch, then the electrical machine has to provide the necessary starting torque beginning from a rotational speed of “zero”. Particularly high starting torques are required e.g. at steep inclines or else at curb edges.
If the electrical machine is embodied as a synchronous machine, for example, then the torque-generating field in the stator is directly coupled to the rotational speed of the machine. As a result, the electric field does not vary at a rotational speed of zero or varies only very slowly at low rotational speeds, but that has the consequence that the field-generating inverter has to provide the required current for a longer time with the same power components. Accordingly, the loading of the power components of the inverter is not uniform in this operating range, such that different temperatures occur at the power components. At higher rotational speeds, by contrast, temperature differences hardly occur since the thermal time constants are considerably greater than the commutation times of the power components.
Since the temperature swings that occur at the power components of the inverter crucially influence the lifetime of these components, it is necessary to limit the temperature swing in the power components. For this purpose, in the event of a limit value for the temperature swing being reached, the current for generating the torque-generating field is reduced and the torque or the power of the electrical machine is thus limited.
EP 1 768 238 A1 discloses a method for limiting the temperature of an output stage of an electric motor, wherein the temperature of at least one bridge component contained in the output stage is determined by virtue of the fact that the phase current flowing through the bridge component is measured, the power loss of the bridge component is calculated from the phase current, the temperature increase arising at the bridge component is determined from the power loss and the temperature of a baseplate of the output stage is added to the temperature increase. If the determined temperature of the bridge component exceeds a specific maximum temperature, the output torque of the electric motor is reduced.